Proteolysis of each of the complement proteins C3-C5 gives rise to amino-terminal cationic fragments with signalling molecules called anaphylatoxins. The most potent of these, C5a, elicits the broadest responses. Considering the components of the inflammatory response as margination and infiltration of leukocytes, release of granule-bound proteolytic enzymes, production of activated oxygen and nitrogen-derived radicals, changes in blood flow and capillary leakage, along with the ability to contract smooth muscle, the C5a molecule is the “complete” pro-inflammatory mediator. At sub-nanomolar to nanomolar levels, the C5a molecule elicits chemotaxis of all myeloid lineages (neutrophils, eosinophils and basophils, macrophages and monocytes), and causes vascular permeability which is markedly potentiated by prostaglandins and circulating leukocytes. Higher nanomolar concentrations elicit degranulation and activation of NADPH oxidase. This breadth of bioactivity contrasts with other inflammatory mediators. C5a is involved in the pathogenesis of various disorders including rheumatoid arthritis, psoriasis, sepsis, reperfusion injury, and adult respiratory distress syndrome (Gerard and Gerard, 1994; Murdoch and Finn, 2000).
The activities of C5a are mediated by the binding of the C5a to its receptor (C5aR). C5aR belongs to the family of seven transmembrane G-protein-coupled receptors. C5aR is a high affinity receptor for C5a, with a Kd of ˜1 nM, and is located on a number of different cell types including leukocytes. The number of receptors per cell is extremely high, up to 200,000 sites per leukocyte. Biological activation of the receptor occurs over the range that saturates binding.
The C5aR structure conforms to the seven transmembrane receptor family, with the extracellular N-terminus being followed by seven transmembrane helices connected by interhelical domains alternating as intracellular and extracellular loops, and ending with an intracellular C-terminal domain. C5aR contains an extended N-terminal extracellular domain. This large N-terminal domain is typical of G-protein coupled receptors which bind peptides including the IL-8 and fMet-Leu-Phe (FMLP) receptor families.
Inhibition of the C5a responses with C5aR antagonists reduces the acute inflammatory response mediated via C5a without affecting other complement components. To this end, C5aR peptide antagonists and anti-C5a receptor antibodies have been previously described (Watanabe et al., 1995; Pellas et al., 1998; Konteatis et al., 1994; Kaneko et al., 1995; Morgan et al., 1993). For example, WO 95/00164 describes antibodies directed against an N-terminal peptide (residues 9-29) of C5aR. WO 03/062278 also describes antibodies directed against C5aR. Three of these mouse antibodies were termed 7F3, 6C12 and 12D4. These antibodies were shown to have excellent properties, such as being very effective at blocking C5a binding to its receptor, stopping C5a-directed migration of neutrophils in vitro, and preventing inflammation in animal models. To control chronic diseases it may be necessary to administer the antibody on successive occasions over months or years. However, one drawback from administering mouse antibodies is that the human immune system may generate its own antibodies directed against the mouse antibody (the HAMA response). The HAMA response can neutralize the mouse antibodies by rapidly clearing them from the blood, thus preventing the mouse antibody from binding to its target. To avoid development of a HAMA response one strategy that has been adopted is to “humanize” the mouse antibody by replacing as many “foreign” residues in the non-epitope binding regions with human sequences.
A major problem of humanization procedures has been a loss of affinity for the antigen (Jones et al., 1986), in some instances as much as 10-fold or more, especially when the antigen is a protein (Verhoeyen et al., 1988). Loss of any affinity is, of course, highly undesirable. At the least, it means that more of the humanized antibody will have to be injected into the patient, at higher cost and greater risk of adverse effects. Even more critically, an antibody with reduced affinity may have poorer biological functions, such as complement lysis, antibody-dependent cellular cytotoxicity, or virus neutralization. Although faced with these difficulties successful humanization of anti-human C5aR antibodies has been described in WO 2009/103113.
A plurality of strategies have been developed over the years to further minimize the risk of any unwanted side reaction from administering antibodies to patients, which includes reducing the likelihood of formation of anti-drug antibodies in the patients by generation of “fully” human antibodies.
Even today identification of antibodies suitable for therapeutic applications is a challenging task. Therefore alternative and/or improved C5aR antagonists which can be used in diagnostic and/or therapeutic methods remains of high interest.